You've probably seen a full moon many times during your life, but have you ever noticed that it always looks exactly the same? The Earth and the moon are tidally locked to each other, which means that the same side of the moon always face Earth. This can happen when you have a small body close to a large body, and gravitational interactions cause the small objects orbit and rotation to synchronize. Let me explain. Intuition tells most people that if you always see the same side of the moon, then the moon must not be rotating on it's axis. But this is not true! You can convince yourself of this by doing a little demo with your hands. Hold up your right hand and make a fist, then point the fingers of your left hand toward it. Now move your left hand around your right, such that the tips of your fingers always point to your right fist. You'll quickly find that you have to rotate your left hand to do that! The moon goes around the Earth once every ~28 days, and it rotates on its axis once every ~28 days as well. This causes the same side of the moon to always face Earth! Planets that are close to their host stars can be tidally locked in the same fashion, and so can two stars. Many moons in our solar system are known to be tidally locked to their host planet, and astronomers speculate that many known exoplanets are tidally locked to their host stars.
